This invention relates to vehicle detectors used to detect the presence or absence of a motor vehicle in an inductive loop embedded in a roadbed. More particularly, this invention relates to a vehicle detector with improved reference tracking.
Vehicle detectors have been used for a substantial period of time to generate information specifying the presence or absence of a vehicle at a particular location. Such detectors have been used at intersections, for example, to supply information used to control the operation of the traffic signal heads, and have also been used to supply control information used in conjunction with automatic entrance and exit gates in parking lots, garages and buildings. A widely used type of vehicle detector employs the principle of period shift measurement in order to determine the presence or absence of a vehicle in or adjacent the inductive loop mounted on or in a roadway. In such systems, a first oscillator, which typically operates in the range from about 10 to about 120 Khz is used to produce a periodic signal in a vehicle detector loop. A second oscillator operating at a much higher frequency is commonly used to generate a sample count signal over a fixed number of loop cycles. The relatively high frequency count signal is typically used to increment a counter, which stores a number corresponding to the sample count at the end of the fixed number of loop cycles. This sample count is compared with a reference count stored in another counter and representative of a previous count in order to determine whether a vehicle has entered or departed the region of the loop in the time period between the previous sample count and the present sample count.
The initial reference value is obtained from one or more initial sample counts and stored in a reference counter. Thereafter, successive sample counts are obtained on a periodic basis, and compared with the reference count. If the two values are essentially equal, the condition of the loop remains unchanged, i.e., a vehicle has not entered or departed the loop. However, if the two numbers differ by at least a threshold amount in a first direction (termed the Call direction), the condition of the loop has changed and may signify that a vehicle has entered the loop. More specifically, in a system in which the sample count has decreased and the sample count has a numerical value less than the reference count by at least a threshold magnitude, this change signifies that the period of the loop signal has decreased (since fewer counts were accumulated during the fixed number of loop cycles), which in turn indicates that the frequency of the loop signal has increased, usually due to the presence of a vehicle in or near the loop. When these conditions exist, the vehicle detector generates a signal termed a Call Signal indicating the presence of a vehicle in the loop.
Correspondingly, if the difference between a sample count and the reference count is greater than a second threshold amount, this condition indicates that a vehicle which was formerly located in or near the loop has left the vicinity. When this condition occurs, a previously generated Call Signal is dropped.
Call Signals are used in a wide variety of applications, including vehicle counting along a roadway or through a parking entrance or exit, vehicle speed between preselected points along a roadway, vehicle presence at an intersection controlled by a traffic control light system, or in a parking stall, and numerous other applications. In all applications, it is necessary to periodically update the reference value so that the vehicle detector can be dynamically adjusted to varying conditions. For example, the loop wire, connecting cables and associated electronic analog circuitry are typically subject to widely varying temperature conditions, which cause the frequency of the loop signal to vary in a somewhat unpredictable manner. If the loop frequency drifts between sample periods by an amount equivalent to the period threshold count in the Call direction, a false call will be generated (since the sample count will be less than the reference count by the threshold value), even though no vehicle has actually entered the loop. This false call can be manifested by a green light in the lane controlled by the detector issuing the false call, even though no vehicle is actually present in that lane. This is clearly highly undesirable since it adversely affects vehicle flow through a controlled traffic system.
In the past, the problem of loop frequency drift has been addressed by a number of techniques. According to one known technique, the reference is slowly adjusted (typically once every two seconds) after taking the sample count by examining the difference between the sample count and the reference and (a) decrementing the reference count by one count when the sample count is less than the reference and (b) incrementing the reference count by one count whenever the sample count exceeds the reference. This technique suffers from several disadvantages. Firstly, while the slow tracking of the loop drift afforded by this approach from the No Call to Call direction is desirable, it is highly undesirable in the opposite direction (i.e., the Call to No Call direction). This is principally due to the fact that, starting with the Call condition, the reference is typically decremented to an artificially low value (typically 100 counts or more below the previous No Call reference value). If the vehicle which generated the call leaves the loop and another vehicle enters the loop, this new call condition will not be detected, since the new sample count will not be less than the current reference value until the reference is incremented by the testing threshold amount (which would take many cycles). As a result, the newly entered vehicle will not be serviced by the traffic control system (i.e., issuance of a green). In an attempt to avoid this disadvantage, a modification of this first technique has been developed which decrements the reference (typically once every two seconds) when the sample count is less than the reference value (the same as the decrementing in the first technique), but which changes the reference to the sample count whenever the sample count exceeds the current reference value. This technique introduces another disadvantage. Specifically, when a noise pulse is generated in the loop which causes the sample count to erroneously rise in value by a significant amount, which is a common occurrence, the new reference value is incorrectly set to an artificially high value. When the noise disappears (typically before the next sample count is taken), the new sample count drops back to the nominal No Call value, which causes a false call to be registered, with the observable disadvantages noted above. Further, since the reference is only decremented (typically once every two seconds), it may take a long period of time (possibly hours) for the reference to be readjusted to the nominal No Call value. During this period of adjustment, false calls are registered for each successive sample, and false greens are issued for the same period of time, which substantially disrupts the traffic control system.
Still further compounding this problem is the fact that an intermittent open loop can also disrupt the reference adjustment process by suddenly raising the loop inductance, which causes a corresponding increase in the sample count. For the case of a shorted loop, the reference value is gradually decremented to the extremely small value of the sample count registered by the shorted loop, during which time a false call will be registered. If the short self-corrects with a vehicle in the loop, the next sample count will exceed that of the invalid reference value and no call will be detected. The new reference will then be adjusted to the sample count obtained with the vehicle. However, since no call will be generated so long as the vehicle remains in the loop, the vehicle will never obtain a green signal, which is highly undesirable.
While other reference tracking routines have been implemented, a need still exists for a reliable tracking routine which minimizes or eliminates false call generation and which results in the reliable generation of valid calls in response to the arrival of a vehicle at the immediate proximity of the loop.